http.h

// Copyright (c) 2017 Sandstorm Development Group, Inc. and contributors
// Licensed under the MIT License:
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

#ifndef KJ_COMPAT_HTTP_H_
#define KJ_COMPAT_HTTP_H_
// The KJ HTTP client/server library.
//
// This is a simple library which can be used to implement an HTTP client or server. Properties
// of this library include:
// - Uses KJ async framework.
// - Agnostic to transport layer -- you can provide your own.
// - Header parsing is zero-copy -- it results in strings that point directly into the buffer
//   received off the wire.
// - Application code which reads and writes headers refers to headers by symbolic names, not by
//   string literals, with lookups being array-index-based, not map-based. To make this possible,
//   the application announces what headers it cares about in advance, in order to assign numeric
//   values to them.
// - Methods are identified by an enum.

#include <kj/string.h>
#include <kj/vector.h>
#include <kj/memory.h>
#include <kj/one-of.h>
#include <kj/async-io.h>

namespace kj {

#define KJ_HTTP_FOR_EACH_METHOD(MACRO) \
  MACRO(GET) \
  MACRO(HEAD) \
  MACRO(POST) \
  MACRO(PUT) \
  MACRO(DELETE) \
  MACRO(PATCH) \
  MACRO(PURGE) \
  MACRO(OPTIONS) \
  MACRO(TRACE) \
  /* standard methods */ \
  /* */ \
  /* (CONNECT is intentionally omitted since it is handled specially in HttpHandler) */ \
  \
  MACRO(COPY) \
  MACRO(LOCK) \
  MACRO(MKCOL) \
  MACRO(MOVE) \
  MACRO(PROPFIND) \
  MACRO(PROPPATCH) \
  MACRO(SEARCH) \
  MACRO(UNLOCK) \
  /* WebDAV */ \
  \
  MACRO(REPORT) \
  MACRO(MKACTIVITY) \
  MACRO(CHECKOUT) \
  MACRO(MERGE) \
  /* Subversion */ \
  \
  MACRO(MSEARCH) \
  MACRO(NOTIFY) \
  MACRO(SUBSCRIBE) \
  MACRO(UNSUBSCRIBE)
  /* UPnP */

#define KJ_HTTP_FOR_EACH_CONNECTION_HEADER(MACRO) \
  MACRO(connection, "Connection") \
  MACRO(contentLength, "Content-Length") \
  MACRO(keepAlive, "Keep-Alive") \
  MACRO(te, "TE") \
  MACRO(trailer, "Trailer") \
  MACRO(transferEncoding, "Transfer-Encoding") \
  MACRO(upgrade, "Upgrade") \
  MACRO(websocketKey, "Sec-WebSocket-Key") \
  MACRO(websocketVersion, "Sec-WebSocket-Version") \
  MACRO(websocketAccept, "Sec-WebSocket-Accept")

enum class HttpMethod {
  // Enum of known HTTP methods.
  //
  // We use an enum rather than a string to allow for faster parsing and switching and to reduce
  // ambiguity.

#define DECLARE_METHOD(id) id,
KJ_HTTP_FOR_EACH_METHOD(DECLARE_METHOD)
#undef DECLARE_METHOD
};

kj::StringPtr KJ_STRINGIFY(HttpMethod method);
kj::Maybe<HttpMethod> tryParseHttpMethod(kj::StringPtr name);

class HttpHeaderTable;

class HttpHeaderId {
  // Identifies an HTTP header by numeric ID that indexes into an HttpHeaderTable.
  //
  // The KJ HTTP API prefers that headers be identified by these IDs for a few reasons:
  // - Integer lookups are much more efficient than string lookups.
  // - Case-insensitivity is awkward to deal with when const strings are being passed to the lookup
  //   method.
  // - Writing out strings less often means fewer typos.
  //
  // See HttpHeaderTable for usage hints.

public:
  HttpHeaderId() = default;

  inline bool operator==(const HttpHeaderId& other) const { return id == other.id; }
  inline bool operator!=(const HttpHeaderId& other) const { return id != other.id; }
  inline bool operator< (const HttpHeaderId& other) const { return id <  other.id; }
  inline bool operator> (const HttpHeaderId& other) const { return id >  other.id; }
  inline bool operator<=(const HttpHeaderId& other) const { return id <= other.id; }
  inline bool operator>=(const HttpHeaderId& other) const { return id >= other.id; }

  inline size_t hashCode() const { return id; }

  kj::StringPtr toString() const;

  void requireFrom(HttpHeaderTable& table) const;
  // In debug mode, throws an exception if the HttpHeaderId is not from the given table.
  //
  // In opt mode, no-op.

#define KJ_HTTP_FOR_EACH_BUILTIN_HEADER(MACRO) \
  MACRO(HOST, "Host") \
  MACRO(DATE, "Date") \
  MACRO(LOCATION, "Location") \
  MACRO(CONTENT_TYPE, "Content-Type")
  // For convenience, these very-common headers are valid for all HttpHeaderTables. You can refer
  // to them like:
  //
  //     HttpHeaderId::HOST
  //
  // TODO(soon): Fill this out with more common headers.

#define DECLARE_HEADER(id, name) \
  static const HttpHeaderId id;
  // Declare a constant for each builtin header, e.g.: HttpHeaderId::CONNECTION

  KJ_HTTP_FOR_EACH_BUILTIN_HEADER(DECLARE_HEADER);
#undef DECLARE_HEADER

private:
  HttpHeaderTable* table;
  uint id;

  inline explicit constexpr HttpHeaderId(HttpHeaderTable* table, uint id): table(table), id(id) {}
  friend class HttpHeaderTable;
  friend class HttpHeaders;
};

class HttpHeaderTable {
  // Construct an HttpHeaderTable to declare which headers you'll be interested in later on, and
  // to manufacture IDs for them.
  //
  // Example:
  //
  //     // Build a header table with the headers we are interested in.
  //     kj::HttpHeaderTable::Builder builder;
  //     const HttpHeaderId accept = builder.add("Accept");
  //     const HttpHeaderId contentType = builder.add("Content-Type");
  //     kj::HttpHeaderTable table(kj::mv(builder));
  //
  //     // Create an HTTP client.
  //     auto client = kj::newHttpClient(table, network);
  //
  //     // Get http://example.com.
  //     HttpHeaders headers(table);
  //     headers.set(accept, "text/html");
  //     auto response = client->send(kj::HttpMethod::GET, "http://example.com", headers)
  //         .wait(waitScope);
  //     auto msg = kj::str("Response content type: ", response.headers.get(contentType));

  struct IdsByNameMap;

public:
  HttpHeaderTable();
  // Constructs a table that only contains the builtin headers.

  class Builder {
  public:
    Builder();
    HttpHeaderId add(kj::StringPtr name);
    Own<HttpHeaderTable> build();

    HttpHeaderTable& getFutureTable();
    // Get the still-unbuilt header table. You cannot actually use it until build() has been
    // called.
    //
    // This method exists to help when building a shared header table -- the Builder may be passed
    // to several components, each of which will register the headers they need and get a reference
    // to the future table.

  private:
    kj::Own<HttpHeaderTable> table;
  };

  KJ_DISALLOW_COPY(HttpHeaderTable);  // Can't copy because HttpHeaderId points to the table.
  ~HttpHeaderTable() noexcept(false);

  uint idCount();
  // Return the number of IDs in the table.

  kj::Maybe<HttpHeaderId> stringToId(kj::StringPtr name);
  // Try to find an ID for the given name. The matching is case-insensitive, per the HTTP spec.
  //
  // Note: if `name` contains characters that aren't allowed in HTTP header names, this may return
  //   a bogus value rather than null, due to optimizations used in case-insensitive matching.

  kj::StringPtr idToString(HttpHeaderId id);
  // Get the canonical string name for the given ID.

private:
  kj::Vector<kj::StringPtr> namesById;
  kj::Own<IdsByNameMap> idsByName;
};

class HttpHeaders {
  // Represents a set of HTTP headers.
  //
  // This class guards against basic HTTP header injection attacks: Trying to set a header name or
  // value containing a newline, carriage return, or other invalid character will throw an
  // exception.

public:
  explicit HttpHeaders(HttpHeaderTable& table);

  KJ_DISALLOW_COPY(HttpHeaders);
  HttpHeaders(HttpHeaders&&) = default;
  HttpHeaders& operator=(HttpHeaders&&) = default;

  void clear();
  // Clears all contents, as if the object was freshly-allocated. However, calling this rather
  // than actually re-allocating the object may avoid re-allocation of internal objects.

  HttpHeaders clone() const;
  // Creates a deep clone of the HttpHeaders. The returned object owns all strings it references.

  HttpHeaders cloneShallow() const;
  // Creates a shallow clone of the HttpHeaders. The returned object references the same strings
  // as the original, owning none of them.

  kj::Maybe<kj::StringPtr> get(HttpHeaderId id) const;
  // Read a header.

  template <typename Func>
  void forEach(Func&& func) const;
  // Calls `func(name, value)` for each header in the set -- including headers that aren't mapped
  // to IDs in the header table. Both inputs are of type kj::StringPtr.

  void set(HttpHeaderId id, kj::StringPtr value);
  void set(HttpHeaderId id, kj::String&& value);
  // Sets a header value, overwriting the existing value.
  //
  // The String&& version is equivalent to calling the other version followed by takeOwnership().
  //
  // WARNING: It is the caller's responsibility to ensure that `value` remains valid until the
  //   HttpHeaders object is destroyed. This allows string literals to be passed without making a
  //   copy, but complicates the use of dynamic values. Hint: Consider using `takeOwnership()`.

  void add(kj::StringPtr name, kj::StringPtr value);
  void add(kj::StringPtr name, kj::String&& value);
  void add(kj::String&& name, kj::String&& value);
  // Append a header. `name` will be looked up in the header table, but if it's not mapped, the
  // header will be added to the list of unmapped headers.
  //
  // The String&& versions are equivalent to calling the other version followed by takeOwnership().
  //
  // WARNING: It is the caller's responsibility to ensure that `name` and `value` remain valid
  //   until the HttpHeaders object is destroyed. This allows string literals to be passed without
  //   making a copy, but complicates the use of dynamic values. Hint: Consider using
  //   `takeOwnership()`.

  void unset(HttpHeaderId id);
  // Removes a header.
  //
  // It's not possible to remove a header by string name because non-indexed headers would take
  // O(n) time to remove. Instead, construct a new HttpHeaders object and copy contents.

  void takeOwnership(kj::String&& string);
  void takeOwnership(kj::Array<char>&& chars);
  void takeOwnership(HttpHeaders&& otherHeaders);
  // Takes overship of a string so that it lives until the HttpHeaders object is destroyed. Useful
  // when you've passed a dynamic value to set() or add() or parse*().

  struct ConnectionHeaders {
    // These headers govern details of the specific HTTP connection or framing of the content.
    // Hence, they are managed internally within the HTTP library, and never appear in an
    // HttpHeaders structure.

#define DECLARE_HEADER(id, name) \
    kj::StringPtr id;
    KJ_HTTP_FOR_EACH_CONNECTION_HEADER(DECLARE_HEADER)
#undef DECLARE_HEADER
  };

  struct Request {
    HttpMethod method;
    kj::StringPtr url;
    ConnectionHeaders connectionHeaders;
  };
  struct Response {
    uint statusCode;
    kj::StringPtr statusText;
    ConnectionHeaders connectionHeaders;
  };

  kj::Maybe<Request> tryParseRequest(kj::ArrayPtr<char> content);
  kj::Maybe<Response> tryParseResponse(kj::ArrayPtr<char> content);
  // Parse an HTTP header blob and add all the headers to this object.
  //
  // `content` should be all text from the start of the request to the first occurrance of two
  // newlines in a row -- including the first of these two newlines, but excluding the second.
  //
  // The parse is performed with zero copies: The callee clobbers `content` with '\0' characters
  // to split it into a bunch of shorter strings. The caller must keep `content` valid until the
  // `HttpHeaders` is destroyed, or pass it to `takeOwnership()`.

  kj::String serializeRequest(HttpMethod method, kj::StringPtr url,
                              const ConnectionHeaders& connectionHeaders) const;
  kj::String serializeResponse(uint statusCode, kj::StringPtr statusText,
                               const ConnectionHeaders& connectionHeaders) const;
  // Serialize the headers as a complete request or response blob. The blob uses '\r\n' newlines
  // and includes the double-newline to indicate the end of the headers.

  kj::String toString() const;

private:
  HttpHeaderTable* table;

  kj::Array<kj::StringPtr> indexedHeaders;
  // Size is always table->idCount().

  struct Header {
    kj::StringPtr name;
    kj::StringPtr value;
  };
  kj::Vector<Header> unindexedHeaders;

  kj::Vector<kj::Array<char>> ownedStrings;

  kj::Maybe<uint> addNoCheck(kj::StringPtr name, kj::StringPtr value);

  kj::StringPtr cloneToOwn(kj::StringPtr str);

  kj::String serialize(kj::ArrayPtr<const char> word1,
                       kj::ArrayPtr<const char> word2,
                       kj::ArrayPtr<const char> word3,
                       const ConnectionHeaders& connectionHeaders) const;

  bool parseHeaders(char* ptr, char* end, ConnectionHeaders& connectionHeaders);

  // TODO(perf): Arguably we should store a map, but header sets are never very long
  // TODO(perf): We could optimize for common headers by storing them directly as fields. We could
  //   also add direct accessors for those headers.
};

class EntropySource {
  // Interface for an object that generates entropy. Typically, cryptographically-random entropy
  // is expected.
  //
  // TODO(cleanup): Put this somewhere more general.

public:
  virtual void generate(kj::ArrayPtr<byte> buffer) = 0;
};

class WebSocket {
  // Interface representincg an open WebSocket session.
  //
  // Each side can send and receive data and "close" messages.
  //
  // Ping/Pong and message fragmentation are not exposed through this interface. These features of
  // the underlying WebSocket protocol are not exposed by the browser-level Javascript API either,
  // and thus applications typically need to implement these features at the application protocol
  // level instead. The implementation is, however, expected to reply to Ping messages it receives.

public:
  virtual kj::Promise<void> send(kj::ArrayPtr<const byte> message) = 0;
  virtual kj::Promise<void> send(kj::ArrayPtr<const char> message) = 0;
  // Send a message (binary or text). The underlying buffer must remain valid, and you must not
  // call send() again, until the returned promise resolves.

  virtual kj::Promise<void> close(uint16_t code, kj::StringPtr reason) = 0;
  // Send a Close message.
  //
  // Note that the returned Promise resolves once the message has been sent -- it does NOT wait
  // for the other end to send a Close reply. The application should await a reply before dropping
  // the WebSocket object.

  virtual kj::Promise<void> disconnect() = 0;
  // Sends EOF on the underlying connection without sending a "close" message. This is NOT a clean
  // shutdown, but is sometimes useful when you want the other end to trigger whatever behavior
  // it normally triggers when a connection is dropped.

  struct Close {
    uint16_t code;
    kj::String reason;
  };

  typedef kj::OneOf<kj::String, kj::Array<byte>, Close> Message;

  virtual kj::Promise<Message> receive() = 0;
  // Read one message from the WebSocket and return it. Can only call once at a time. Do not call
  // again after EndOfStream is received.
};

class HttpClient {
  // Interface to the client end of an HTTP connection.
  //
  // There are two kinds of clients:
  // * Host clients are used when talking to a specific host. The `url` specified in a request
  //   is actually just a path. (A `Host` header is still required in all requests.)
  // * Proxy clients are used when the target could be any arbitrary host on the internet.
  //   The `url` specified in a request is a full URL including protocol and hostname.

public:
  struct Response {
    uint statusCode;
    kj::StringPtr statusText;
    const HttpHeaders* headers;
    kj::Own<kj::AsyncInputStream> body;
    // `statusText` and `headers` remain valid until `body` is dropped or read from.
  };

  struct Request {
    kj::Own<kj::AsyncOutputStream> body;
    // Write the request entity body to this stream, then drop it when done.
    //
    // May be null for GET and HEAD requests (which have no body) and requests that have
    // Content-Length: 0.

    kj::Promise<Response> response;
    // Promise for the eventual respnose.
  };

  virtual Request request(HttpMethod method, kj::StringPtr url, const HttpHeaders& headers,
                          kj::Maybe<uint64_t> expectedBodySize = nullptr) = 0;
  // Perform an HTTP request.
  //
  // `url` may be a full URL (with protocol and host) or it may be only the path part of the URL,
  // depending on whether the client is a proxy client or a host client.
  //
  // `url` and `headers` need only remain valid until `request()` returns (they can be
  // stack-allocated).
  //
  // `expectedBodySize`, if provided, must be exactly the number of bytes that will be written to
  // the body. This will trigger use of the `Content-Length` connection header. Otherwise,
  // `Transfer-Encoding: chunked` will be used.

  struct WebSocketResponse {
    uint statusCode;
    kj::StringPtr statusText;
    const HttpHeaders* headers;
    kj::OneOf<kj::Own<kj::AsyncInputStream>, kj::Own<WebSocket>> webSocketOrBody;
    // `statusText` and `headers` remain valid until `webSocketOrBody` is dropped or read from.
  };
  virtual kj::Promise<WebSocketResponse> openWebSocket(
      kj::StringPtr url, const HttpHeaders& headers);
  // Tries to open a WebSocket. Default implementation calls send() and never returns a WebSocket.
  //
  // `url` and `headers` need only remain valid until `openWebSocket()` returns (they can be
  // stack-allocated).

  virtual kj::Promise<kj::Own<kj::AsyncIoStream>> connect(kj::StringPtr host);
  // Handles CONNECT requests. Only relevant for proxy clients. Default implementation throws
  // UNIMPLEMENTED.
};

class HttpService {
  // Interface which HTTP services should implement.
  //
  // This interface is functionally equivalent to HttpClient, but is intended for applications to
  // implement rather than call. The ergonomics and performance of the method signatures are
  // optimized for the serving end.
  //
  // As with clients, there are two kinds of services:
  // * Host services are used when talking to a specific host. The `url` specified in a request
  //   is actually just a path. (A `Host` header is still required in all requests, and the service
  //   may in fact serve multiple origins via this header.)
  // * Proxy services are used when the target could be any arbitrary host on the internet, i.e. to
  //   implement an HTTP proxy. The `url` specified in a request is a full URL including protocol
  //   and hostname.

public:
  class Response {
  public:
    virtual kj::Own<kj::AsyncOutputStream> send(
        uint statusCode, kj::StringPtr statusText, const HttpHeaders& headers,
        kj::Maybe<uint64_t> expectedBodySize = nullptr) = 0;
    // Begin the response.
    //
    // `statusText` and `headers` need only remain valid until send() returns (they can be
    // stack-allocated).
  };

  virtual kj::Promise<void> request(
      HttpMethod method, kj::StringPtr url, const HttpHeaders& headers,
      kj::AsyncInputStream& requestBody, Response& response) = 0;
  // Perform an HTTP request.
  //
  // `url` may be a full URL (with protocol and host) or it may be only the path part of the URL,
  // depending on whether the service is a proxy service or a host service.
  //
  // `url` and `headers` are invalidated on the first read from `requestBody` or when the returned
  // promise resolves, whichever comes first.

  class WebSocketResponse: public Response {
  public:
    virtual kj::Own<WebSocket> acceptWebSocket(const HttpHeaders& headers) = 0;
    // Accept and open the WebSocket.
    //
    // `headers` need only remain valid until acceptWebSocket() returns (it can be stack-allocated).
  };

  virtual kj::Promise<void> openWebSocket(
      kj::StringPtr url, const HttpHeaders& headers, WebSocketResponse& response);
  // Tries to open a WebSocket. Default implementation calls request() and never returns a
  // WebSocket.
  //
  // `url` and `headers` are invalidated when the returned promise resolves.

  virtual kj::Promise<kj::Own<kj::AsyncIoStream>> connect(kj::StringPtr host);
  // Handles CONNECT requests. Only relevant for proxy services. Default implementation throws
  // UNIMPLEMENTED.
};

kj::Own<HttpClient> newHttpClient(HttpHeaderTable& responseHeaderTable, kj::Network& network,
                                  kj::Maybe<kj::Network&> tlsNetwork = nullptr,
                                  kj::Maybe<EntropySource&> entropySource = nullptr);
// Creates a proxy HttpClient that connects to hosts over the given network.
//
// `responseHeaderTable` is used when parsing HTTP responses. Requests can use any header table.
//
// `tlsNetwork` is required to support HTTPS destination URLs. Otherwise, only HTTP URLs can be
// fetched.
//
// `entropySource` must be provided in order to use `openWebSocket`. If you don't need WebSockets,
// `entropySource` can be omitted. The WebSocket protocol uses random values to avoid triggering
// flaws (including security flaws) in certain HTTP proxy software. Specifically, entropy is used
// to generate the `Sec-WebSocket-Key` header and to generate frame masks. If you know that there
// are no broken or vulnerable proxies between you and the server, you can provide an dummy entropy
// source that doesn't generate real entropy (e.g. returning the same value every time). Otherwise,
// you must provide a cryptographically-random entropy source.

kj::Own<HttpClient> newHttpClient(HttpHeaderTable& responseHeaderTable, kj::AsyncIoStream& stream,
                                  kj::Maybe<EntropySource&> entropySource = nullptr);
// Creates an HttpClient that speaks over the given pre-established connection. The client may
// be used as a proxy client or a host client depending on whether the peer is operating as
// a proxy.
//
// Note that since this client has only one stream to work with, it will try to pipeline all
// requests on this stream. If one request or response has an I/O failure, all subsequent requests
// fail as well. If the destination server chooses to close the connection after a response,
// subsequent requests will fail. If a response takes a long time, it blocks subsequent responses.
// If a WebSocket is opened successfully, all subsequent requests fail.
//
// `entropySource` must be provided in order to use `openWebSocket`. If you don't need WebSockets,
// `entropySource` can be omitted. The WebSocket protocol uses random values to avoid triggering
// flaws (including security flaws) in certain HTTP proxy software. Specifically, entropy is used
// to generate the `Sec-WebSocket-Key` header and to generate frame masks. If you know that there
// are no broken or vulnerable proxies between you and the server, you can provide an dummy entropy
// source that doesn't generate real entropy (e.g. returning the same value every time). Otherwise,
// you must provide a cryptographically-random entropy source.

kj::Own<HttpClient> newHttpClient(HttpService& service);
kj::Own<HttpService> newHttpService(HttpClient& client);
// Adapts an HttpClient to an HttpService and vice versa.

kj::Own<WebSocket> newWebSocket(kj::Own<kj::AsyncIoStream> stream,
                                kj::Maybe<EntropySource&> maskEntropySource);
// Create a new WebSocket on top of the given stream. It is assumed that the HTTP -> WebSocket
// upgrade handshake has already occurred (or is not needed), and messages can immediately be
// sent and received on the stream. Normally applications would not call this directly.
//
// `maskEntropySource` is used to generate cryptographically-random frame masks. If null, outgoing
// frames will not be masked. Servers are required NOT to mask their outgoing frames, but clients
// ARE required to do so. So, on the client side, you MUST specify an entropy source. The mask
// must be crytographically random if the data being sent on the WebSocket may be malicious. The
// purpose of the mask is to prevent badly-written HTTP proxies from interpreting "things that look
// like HTTP requests" in a message as being actual HTTP requests, which could result in cache
// poisoning. See RFC6455 section 10.3.

struct HttpServerSettings {
  kj::Duration headerTimeout = 15 * kj::SECONDS;
  // After initial connection open, or after receiving the first byte of a pipelined request,
  // the client must send the complete request within this time.

  kj::Duration pipelineTimeout = 5 * kj::SECONDS;
  // After one request/response completes, we'll wait up to this long for a pipelined request to
  // arrive.
};

class HttpServer: private kj::TaskSet::ErrorHandler {
  // Class which listens for requests on ports or connections and sends them to an HttpService.

public:
  typedef HttpServerSettings Settings;

  HttpServer(kj::Timer& timer, HttpHeaderTable& requestHeaderTable, HttpService& service,
             Settings settings = Settings());
  // Set up an HttpServer that directs incoming connections to the given service. The service
  // may be a host service or a proxy service depending on whether you are intending to implement
  // an HTTP server or an HTTP proxy.

  kj::Promise<void> drain();
  // Stop accepting new connections or new requests on existing connections. Finish any requests
  // that are already executing, then close the connections. Returns once no more requests are
  // in-flight.

  kj::Promise<void> listenHttp(kj::ConnectionReceiver& port);
  // Accepts HTTP connections on the given port and directs them to the handler.
  //
  // The returned promise never completes normally. It may throw if port.accept() throws. Dropping
  // the returned promise will cause the server to stop listening on the port, but already-open
  // connections will continue to be served. Destroy the whole HttpServer to cancel all I/O.

  kj::Promise<void> listenHttp(kj::Own<kj::AsyncIoStream> connection);
  // Reads HTTP requests from the given connection and directs them to the handler. A successful
  // completion of the promise indicates that all requests received on the connection resulted in
  // a complete response, and the client closed the connection gracefully or drain() was called.
  // The promise throws if an unparseable request is received or if some I/O error occurs. Dropping
  // the returned promise will cancel all I/O on the connection and cancel any in-flight requests.

private:
  class Connection;

  kj::Timer& timer;
  HttpHeaderTable& requestHeaderTable;
  HttpService& service;
  Settings settings;

  bool draining = false;
  kj::ForkedPromise<void> onDrain;
  kj::Own<kj::PromiseFulfiller<void>> drainFulfiller;

  uint connectionCount = 0;
  kj::Maybe<kj::Own<kj::PromiseFulfiller<void>>> zeroConnectionsFulfiller;

  kj::TaskSet tasks;

  HttpServer(kj::Timer& timer, HttpHeaderTable& requestHeaderTable, HttpService& service,
             Settings settings, kj::PromiseFulfillerPair<void> paf);

  kj::Promise<void> listenLoop(kj::ConnectionReceiver& port);

  void taskFailed(kj::Exception&& exception) override;
};

// =======================================================================================
// inline implementation

inline void HttpHeaderId::requireFrom(HttpHeaderTable& table) const {
  KJ_IREQUIRE(this->table == nullptr || this->table == &table,
      "the provided HttpHeaderId is from the wrong HttpHeaderTable");
}

inline kj::Own<HttpHeaderTable> HttpHeaderTable::Builder::build() { return kj::mv(table); }
inline HttpHeaderTable& HttpHeaderTable::Builder::getFutureTable() { return *table; }

inline uint HttpHeaderTable::idCount() { return namesById.size(); }

inline kj::StringPtr HttpHeaderTable::idToString(HttpHeaderId id) {
  id.requireFrom(*this);
  return namesById[id.id];
}

inline kj::Maybe<kj::StringPtr> HttpHeaders::get(HttpHeaderId id) const {
  id.requireFrom(*table);
  auto result = indexedHeaders[id.id];
  return result == nullptr ? kj::Maybe<kj::StringPtr>(nullptr) : result;
}

inline void HttpHeaders::unset(HttpHeaderId id) {
  id.requireFrom(*table);
  indexedHeaders[id.id] = nullptr;
}

template <typename Func>
inline void HttpHeaders::forEach(Func&& func) const {
  for (auto i: kj::indices(indexedHeaders)) {
    if (indexedHeaders[i] != nullptr) {
      func(table->idToString(HttpHeaderId(table, i)), indexedHeaders[i]);
    }
  }

  for (auto& header: unindexedHeaders) {
    func(header.name, header.value);
  }
}

}  // namespace kj

#endif // KJ_COMPAT_HTTP_H_